Question about modifying boat plans

[Thickchesthair]

Long time lurker, first time poster here.

I have read a bunch on building your own boat following plans and am interested in trying a Crackerbox. I have read that you can usually lengthen a boat by up to 10% without affecting the stability etc, but you shouldn't widen the beam.

My question is - can you basically blow up the whole plan by X% and still retain a good ride? For example - if the boats length is 15 feet and the beam is 6 feet (which a standard crackerbox is), can you add 33% to everything and make it 20 feet long with an 8 foot beam? It would be the exact same shape, just bigger in every way.

Cheers,
Derek

[viking north]

There was a big discussion on that just recently. Not sure of the thread but maybe someone will jump in with it. Meanwhile I'll do a quick search and see if i can locate it.eo.

[Thickchesthair]

Thanks. I did search, but I guess the keywords I used missed it. I'll check through again.

Cheers

[viking north]

possible it was a side line on another thread topic I just scanned several back pages but no luck.--Geo,

[viking north]

Found it "Can you resize a design without complications" found it by searching under "scaling up"--Geo.

[Thickchesthair]

Thanks! I'll search that and read up on it!

Cheers,
Derek

[Thickchesthair]

So after reading, it seems that my weights would be way off. I just want to make sure I am doing the math correctly though...

I've read that the hull on the 15' weighs in at around 500lbs, so I figure with 2 passengers (combined weight of 300), engine etc, I would total about 1200lbs. Is that a fair estimate? If so....

I would be going from 15' to 20', that's a 0.75:1 ratio. I used the weight scaling fomula I found in the thread that Viking North pointed to which was (weight / ratio^3) which is:

= 1200 / (0.75 x 0.75 x 0.75)
= 1200 / 0.421875
= 2845 lbs

So am I correct in thinking that the boat (if scaled to from 15' to 20') would have to weigh in at 2845lbs with 2 occupants in order to act the same?

[sorenfdk]

Basically, you shouldn't mess around with existing designs if you haven't got the background necessary, that is if you're not a yacht designer, naval architect or experienced boatbuilder.
Have you thought about the scantlings? Figuring out length and beam and weight is all very well, but how would you figure out the thickness of the plating or the dimensions and spacing of the stiffeners?
You might get away with using the same scantlings if you're only increasing the dimensions with max. 10% (personally, I wouldn't do it!) If you increase by more than that, you're in reality talking about a new design.
I'm not saying this to be negative, but why bother building your own stretched version of an existing design just to see it fall apart when it encounters the first waves?
Lives could be at stake!

[masalai]

Ask the designer and get their approval, else all warranty, certification and so on is null & void...

[BertKu]

Originally Posted by viking north

There was a big discussion on that just recently. Not sure of the thread but maybe someone will jump in with it. Meanwhile I'll do a quick search and see if i can locate it.eo.

Hi Viking, yes it was me who had that heated discussion on this topic. I personally do not believe to blow a design up. But after all my calculations and visit to Wageningen, the problems and advantages were clearly spelled out.
a) the designer has calculated various weights and thickness material. Where do you get the proportionally changed sizes material from? The mistakes one can make and the difficulties one has to locate and get the material from, is mind boggling.
b) You have also the problem that you cannot present a complete set of drawings to the Port authorities for approval, registration and need to invest money again to get a NA to re-calculate.
c) to reduce it by a small margin has been accepted by most authorities, but not increasing.
d) I personally, for my own project, I will make a reduced one with a small percentage. But I have gone through the trouble to calculate every bit of it. Lots of work, but it can be done. Bear in mind, that I use the bases of those original PAID for drawings, to make something different. In Derek’s case he just want to blow it up. The likely hood that it will be a financial success is small. Special if he one day like to sell the boat, except if he is able to produce the calculations.
My view? I would not do it if I was Derek. Except if you have years of time, lots of money to waste and it is your hobby to bugger around with experimenting various options.
Bert

[Thickchesthair]

Believe me when I say that I am not trying to argue anyone's points here. In my last post I was pointing out that I had already run into a huge problem without even doing much work/math.The boat would have to be a boulder in order to work properly, and that wasn't even taking into account the new sizing of structural materials used etc.

I'm just going to look around for a similar design to the crackerbox, but in the 20-22' range.

[BertKu]

Originally Posted by Thickchesthair

Believe me when I say that I am not trying to argue anyone's points here. In my last post I was pointing out that I had already run into a huge problem without even doing much work/math.The boat would have to be a boulder in order to work properly, and that wasn't even taking into account the new sizing of structural materials used etc.

I'm just going to look around for a similar design to the crackerbox, but in the 20-22' range.

Hi Derek,

You are wiser then me. I fully believe you, that you don't feel like disputing some good advise on this forum and are looking around, for an equal suitable design, without having to modify anything.

In my case it was different. There was none. Thus I had to prove to myself that a reduction was not going to endager anybody's life, nor my own.
Bert

[Lurvio]

Thickchesthair

I've had similar thoughts myself (a rear cockpit runabout ~20ft) and my thinking is to pick a bigger design (like the GlenL Riviera) and build the topsides differently. It should be pretty easy the keep the weights about the same, so you won't affect the attitude of the boat too much. Should be cool to look over the 15 ft bow deck.

Lurvio

[Wavewacker]

I agree with the advice I obtained on the same issue, if I have to make a change that exceeds 10% of something, I have the wrong boat to begin with.
The roof on many designs will effect the boats righting ability, wind draft and could be structural element of other elements in certain situations, like a broadside hit, trailering or support while dry docked, as a roof could help bridge loads. Designs are made three demensionally, IMHO, with various considerations and any change needs to be assessed accordingly. I considered building a sharpie and the original plan shows a higher, squared roof, my change would be a little longer, lower and more rounded on top, like that found on a canal boat rather than a victorian cruiser. Such a change appears, initially to be sound, but I'm still looking for other designs that fit my requirements. If I can't find what I want, then I will be seeking specific advice (and blessings) to do so. I have not built a boat like that before, I have built forumla racing cars and houses, but I'm not an engineer and after 50 years of being around and owning various boats, I'm learning about boat construction and design!